Materials with different compositions exhibit slight, but measurable, differences in their absorption of infrared radiation. Thus, infrared analysis can be used to determine chemical composition and corresponding physical properties of materials.
Infrared measures the absorbance of functional groups, whose number and types are determined by the chemical composition of the material. Infrared is able to estimate physical properties because the physical properties are related to the chemical composition.
Infrared analysis is a secondary analytical technique that is calibrated against a direct technique (Primary Reference Method). Infrared analysis requires a training set of the material (Calibration Set of Samples) for which both the infrared spectra (Calibration Set of Spectra) and Primary Reference Method measurements of properties of interest are obtained.
As shown in FIG. 1, the prior art uses regression mathematics to correlate the infrared spectra of the Original Calibration Set of Spectra to measurements of the Calibration Set of Samples of the material obtained by the Primary Reference Method. The resulting regression equations provide the means to estimate the properties of specimens of the material, for which Primary Reference Method measurements have not been made, from the infrared spectra of the specimens. These regression equations of the prior art do not compensate for instabilities in the infrared instrument that generates the spectra.
Infrared analysis currently is a common method for analyzing agricultural products. For example, it can be used to analyze the protein content of wheat and other grains. In recent years, it also has been applied in the petrochemical industry for analysis of both chemical composition (aromatic and saturates content) and physical properties (octane number, density, vapor pressure) of hydrocarbons including gasoline.
Octane numbers represent a gasoline's ability to resist knocking when used as a fuel in a spark-ignition engine. The higher the octane number, the more resistant the fuel is to knocking. The name "octane number" comes from an empirical scale developed in the 1930's wherein pure iso-octane was defined as 100, normal heptane as 0 and mixtures of the two were used to define intermediate octane numbers.
A spark-ignition engine achieves its maximum power and fuel efficiency when it operates just on the edge of knocking. Knocking is the uncontrolled explosion of the last portion of the fuel-air mixture in the cylinder. Antiknock compounds slightly retard combustion and thus prevent knocking. Branched alkanes such as iso-octane are less likely to detonate and, therefore, have higher octane numbers while straight chain alkanes, such as normal heptane, are more likely to detonate and have lower octane numbers. Aromatics are less likely to detonate and, therefore, have high octane numbers.
Under conditions of normal combustion, the spark plug ignites a wave of flame which moves smoothly and uniformly away from the spark plug to the other side of the combustion chamber and causes a uniform buildup of pressure that firmly pushes the piston down and turns the crankshaft. When knocking occurs, however, the last portion of the fuel (farthest from the spark plug) ignites all at once creating a pressure pulse similar to striking the piston with a hammer and causing the "pinging" sound associated with knock. Less of the energy of the fuel is transferred into motion of the piston and more into heat and deformation and damage to the piston or cylinder.
Currently, large one-cylinder engines are used as the Primary Reference Method to determine the octane ratings of gasolines by comparing the intensity of the knock of a gasoline to that of a standard fuel mixture (e.g. a mixture of iso-octane and normal heptane) and adjusting the compression ratio until the knock intensity of the gasoline is the same as it was for the standard fuel mixture prior to the adjustment. By using a standard reference table, the amount of adjustment of the compression ratio (that was necessary to match the loudness of the gasoline knock to that of the standard fuel mixture) can be related to the difference between the octane number of the gasoline and the octane number of the standard fuel.
There are two types of knock-engine-measured octane ratings, research (ASTM D2699) and motor (ASTM D2700), that correspond to different operating conditions of the engine. Motor octane ratings are performed at higher speed and temperature than research octane ratings. Because the motor test is more severe, motor octane ratings are lower than research octane ratings for the same fuel.
The pump octane number (PON) is the,average of the research octane number (RON) and the motor octane number (MON) and is the number posted at the gas station pump. It is intended to reflect the performance of the fuel under conditions midway in severity between that of the motor and research octane tests.
Recently, there has been considerable interest in finding alternatives to using the octane-rating engines while retaining the accuracy of this direct measurement technique. Near-infrared analysis has received considerable attention as an alternative. Near-infrared analysis of gasoline correlates the physical property of octane number to the near-infrared spectra of the gasoline. Such correlation is possible because the near-infrared spectrum of a gasoline reflects the gasoline's chemical composition such as degree of branching or aromaticity which affects the gasoline's octane number.
One obstacle to the more widespread use of near-infrared analysis for estimating octane number has been stability problems with the instruments that generate the infrared spectra. The regression equations obtained from infrared analysis are often so sensitive that an equation developed on one infrared instrument cannot be used without modification on another instrument, even another instrument from the same manufacturer. The regression equations often include subtle instrument-response characteristics as well as the main sample spectral characteristics. This is especially true for infrared regression equations for physical properties such as the octane number of gasoline.
Even when the analysis is restricted to a single infrared instrument, problems can develop over time. After weeks or months of use, an infrared instrument's response function can change enough to cause estimations based on that instrument's spectra to wander outside the acceptable range of uncertainty.
Instrument stability is a major issue in the infrared community. Considerable effort has gone into improving the instruments resulting in a reduction of instabilities in most instruments but not elimination of them. The most common changes in instrument response over time are wavelength drifts (or jumps), transmittance-baseline changes and increases in absorbance noise. The present invention addresses these instrument stability problems.